If you have a factory-built vehicle (motorcycle, car, truck) and have a title for it, is there a limit on how much you can structurally modify it, before the original title is no longer valid and you have to go through the process of applying for a new title as an owner-built vehicle?

One example would be the Riley Tri-Magnum, which was featured in Mechanix Illustrated many years ago.  The plans explain how to remove the forks from a motorcycle and replace them with a frame that connects to a VW Bug front axle and supports a pod with side-by-side seating.  It gets into a bit of a grey area on whether or not it is still a motorcycle, because the riders are enclosed by the pod (still no seatbelts, though) and it has a steering wheel instead of handlebars, so I can see it being a problem due to the possibility of no longer legally being a motorcycle under state law.  If the pod was built as a single seat roadster layout (something like an Indy car, from the seat forward) and it used handlebars instead of a steering wheel, it would fit the 'feet forward, reverse trike' motorcycle configuration and likely avoid the question of whether or not it is still legally a motorcycle, but would a new title be necessary?

There are kit cars that involve buying or building a new frame and body, then using drivetrain and suspension components from a donor vehicle, and those would be (as far as I know) considered to be owner-built vehicles requiring an application for a title, but what about the kit cars that put a body on a VW chassis or Pontiac Fiero chassis?  There have even been kits that modified the VW and Fiero chassis to change the wheelbase (chopping a VW pan or stretching the Fiero chassis).

Anybody know if the state of Tennessee has guidelines on just how far a titled vehicle can be modified, before the original title is no longer valid?